a.) Field of the Invention
The present invention relates generally to static structures. More specifically, it relates to concrete panel structures in a form which is useful for trusses, structural floors, or for use in bridge decks. The present invention also relates to methods of producing concrete panels for use in trusses, structural floors and bridge deck structures.
b.) Description of the Prior Art
Typically, traffic bearing floors on bridges are constructed using concrete bridge deck panels supported by a specifically designed substructure. Such concrete panels are normally at least six inches thick, and are continuous over at least a pair of separated support members, such as beams, which beams extend longitudinally in the same direction as what is defined herein as the length of the panels bridge span. State-of-the-art concrete bridge deck panel construction has traditionally been comprised of a slab constructed of one layer or more than one layer of concrete having a xe2x80x9cflexural reinforcing structurexe2x80x9d distributed throughout the concrete layer. Such a xe2x80x9cflexural reinforcing structurexe2x80x9d is generally in the form of a matrix of overlapping steel reinforcing bars (re-bars) or steel strands, which are spaced from both the upper surface and the lower surface of the concrete deck panel. In accordance with traditional practice, this flexural reinforcing structure is included in the concrete for the purpose of carrying bending moment tension stresses which are placed on the concrete panel due to loading and unloading of the top surface, for example, by the passage of vehicles on, or adjacent to, the top surface of the panel.
It has traditionally been believed that structural flexural reinforcing material such as steel reinforcing bars (re-bars), are required throughout the concrete of such a panel, and especially in groups in the top and bottom halves of the panel near both the top surface and bottom surface of the panel. In the current state-of-the-art, it is believed to be necessary to use both top and bottom structural flexural reinforcing material re-bars in order to restrain cracking of the top surface and of the bottom surface due to applied loads. The traditional art of bridge deck design and construction has been governed by AASHTO (American Association of State Highway and Transportation Officials). The 1989 Edition of the AASHTO Standard Specification for Highway Bridges specifies the minimum thickness of bridge deck 6.5 inches.
The lower group of flexural reinforcing material in the bottom half of a bridge deck panel normally consists of a first plurality of re-bars which form a layer. This first plurality of re-bars are transverse to both the length dimension of the panel and to the load-carrying beams on which the panel is supported. For structural purposes, this lower layer of transverse flexural materials (re-bar) carries the positive moment tensile stresses which are applied to the panel. A second lower layer of flexural reinforcing material, normally consisting of a second plurality of re-bars which are parallel to both the length dimension of the panel and to the load-carrying, support beams (and transverse to the first lower layer of re-bars) is located directly above the first lower layer of re-bars. For structural purposes this second lower layer of flexural reinforcing material re-bars distributes the bending moment loads which are applied to the panel longitudinally. Both lower layers of flexural reinforcing material re-bars provide control of temperature and shrinkage cracking at the lower surface of the panel as the minimum amount required for temperature and shrinkage reinforcement is less than the minimum required amount of flexural reinforcing for reinforced concrete. Under current codes, for most support beam spacings, which are up to about eleven feet apart, the longitudinal bottom group of flexural reinforcing material constitutes from about one-half to about two-thirds of the main reinforcement of the panel. The two lower layers of flexural reinforcing material are usually joined together, for example with wire, to form a mat or matrix.
Further, in accordance with current practice, another group of main flexural reinforcing material is located in the top half of the panel near the upper surface of the concrete panel. It normally consists of a first upper layer comprised of a plurality of flexural reinforcing materials, normally re-bar, which are designed to carry the negative moment tensile stresses which are applied to the panel, and a second upper layer normally immediately below the first upper layer and oriented transversely to the first upper layer comprised of a plurality of flexural reinforcing material which are intended for control of temperature change and concrete volume shrinkage cracking and to hold the uppermost flexural reinforcing materials in position during concrete placement. Both upper layers of flexural reinforcing material re-bars are intended to provide control of temperature shrinkage cracking at the upper surface of the panel. In addition to their function as flexural reinforcing, the first upper layer of re-bars is intended to provide control of temperature and shrinkage cracking at the upper surface of the panel. The upper group of flexural reinforcing materials is also usually in the form of a mat or matrix, which matrix is sized and oriented substantially identical to, and also parallel to, the flexural reinforcing matrix group in the lower half of the concrete panel.
Flexural reinforcing materials composed of steel re-bars, which re-bars are not coated or connected to a sacrificial anode, corrode readily when exposed to thawing salts and other corrosive elements, and even to ordinary water.
Despite the above described traditional flexural reinforcing of concrete bridge deck panel structures, concrete bridge deck panels have been found to deteriorate rapidly and to require costly rehabilitation or replacement from time-to-time. It has been recently estimated, for example, that the use of thawing salts on bridges in the United States causes $1.6 billion dollars worth of damage annually. Similar problems exist outside of the United States. Thus, there is a world-wide need to reduce the deterioration of concrete bridge deck panels without reducing the ability of the bridge deck panels to resist moment stresses imposed thereon by traffic loads.
It has been determined that much of the deterioration of concrete bridge deck panels is actually attributable to the corrosion of the traditional flexural reinforcing bars in the upper half of such bridge deck panels. It had been the common practice, until the late 1960""s, to construct most concrete bridge deck panels over girder bridges with the bottom flexural reinforcing bars bent up over the supporting elements, such as beams or girders. Because of their shape, such bent flexural strength reinforcing bars are sometimes referred to as xe2x80x9ccrank bars,xe2x80x9d because they resemble crankshafts. In the late 1960""s the use of thawing salts on roads became quite prevalent. Subsequently the use of a greater amount of continuous straight flexural reinforcing re-bars in the top half of the concrete panel replaced the use of crank bars, because it was found to be more cost efficient to use more flexural reinforcing bars in the top half, than to bend and place crank bars in the lower half. This practice also helped maintain the proper position of the bars in the top mat. As a result, this practice substantially increased the amount of corrodible steel re-bar material in the top of the deck panel. Bridge deck panels of this era were also constructed with only about 1.5 inches (3.8 cm) of protective concrete cover over the continuous straight top bars or re-bars.
During the early 1970""s, the protective concrete cover over the top re-bars was generally increased to greater than about 2 inches (5.1 cm). At the same time, construction practices were improved so that reduction of the thickness of the top cover during panel placement, was avoided. It was believed that the additional thickness of the top concrete cover would limit or slow cracking of the top surface, and thus lengthen the time that it took for chlorides from thawing salts and other corrosive elements to penetrate to the level of the re-bars contained in the upper portion of the concrete panel.
The understanding that chlorides from thawing salts and other corrosive materials corrode the re-bars in the upper half of the concrete panel, and thus constitute the source of significant cracking and deterioration of the top surface of the bridge deck panel is important to the present invention, as set forth below.
Surprisingly, the additional thickness of concrete top cover included in bridge deck panel designs during the 1970""s did not extend bridge deck panel life significantly. Subsequently, in most jurisdictions in which thawing salt is used, it became the practice to take steps to make bridge deck panels more impervious to the penetration of moisture, salt and other corrosive materials. It was believed that if the salt and other corrosive materials could not reach the re-bars in the upper half of the concrete layer, that the corrosion problem would be solved. Consequently, richer concrete mixes which were known to be more impervious to salts than traditional concrete mixes were utilized, and as a result the use of concrete having greater load bearing strengths then became standard practice. However, the use of richer concrete mixes led to yet another problem, in that such concrete exhibited increased shrinkage characteristics.
It is believed that the increased shrinkage of the used richer concrete mixes may be primarily, or at least partly, responsible for additional cracks developing in the top surface of the concrete in recently constructed concrete deck panel structures. Of course, such cracks allow thawing salts and other corrosive materials to reach the corrodible re-bars in the upper half of the concrete panel and cause them to corrode, and thereby cause deterioration of the panel.
It is also known that cracking in the upper surface of concrete bridge deck panels can be avoided by careful control of the concrete mix and by concrete placement techniques. However, to be successful, such a strategy requires careful selection and proportioning of concrete mix materials, and meticulous concrete placement and curing practice. These techniques have not been widely employed as part of a bridge deck construction strategy because it was thought that control of negative moment stresses in the upper surface of bridge decks was the dominate requirement for the restraint of cracking in the upper surface.
Several barrier technologies have been developed to stop or limit corrosion of flexural reinforcing re-bar materials which are located in the top half of concrete bridge deck panels from contact with thawing salts and other corrosive materials. Such barrier technologies include, for example, surface membranes, dense concrete, latex modified concrete, epoxy coated re-bars and the like. These barrier systems have had only moderate success.
Epoxy coated re-bars have proven to provide the most satisfactory corrosion protection, since such epoxy coatings, if continuous, virtually eliminate all actual contact between the re-bars and the thawing salts or other corrosive materials. However, it will be recalled that such re-bars are normally installed as matrices, which are often connected by tie wires and chains to the re-bar matrix in the lower portion of the concrete. The connecting tie wires and chains are usually electrically conductive. It has been found that placing a matrix of epoxy coated re-bars in the upper half of the concrete panel into electrical connection with the uncoated matrix of re-bars in the lower half of the panel allows an electrical half-cell to develop which encourages corrosion of the upper matrix of epoxy coated flexural reinforcing material. Additionally, epoxy coating re-bars apparently do not bond with the concrete in the panel as well as uncoated re-bars. Therefore, when epoxy coated re-bars are used in the top half of a concrete panel, once surface cracking is initiated, the length and width of cracks in the top surface tend to be larger than they would be had uncoated re-bars been used.
Waterproofing membrane barrier systems have been coated on the top surface of concrete panels. One potential problem with such waterproofing membrane barrier systems is that, should any moisture manage to migrate or collect below the membrane, it creates a closed, moisture retaining environment in which corrosion can occur, whether or not salts or other corrosive materials are present. Furthermore, such barrier systems may conceal the deterioration of the top of the concrete from view, thereby delaying remedial maintenance until deterioration has become quite severe.
The above sequence of developments in the prior art of concrete bridge deck panels has been extremely costly. The combined effects of the additional thickness of the concrete, the use of epoxy coated re-bars in the upper portion of the bridge deck panel, the coating of waterproofing membrane systems on the top surface, and the increased girder weight necessary to carry the greater deadload of thicker deck panels, have all increased the cost of bridge deck panel systems by as much as about 30% to about 50%. Furthermore, despite the recognition of the problems caused by the corrosion of upper half flexural reinforcing re-bar,and the various technologies which have been developed to combat them, and even with the increased cost, deterioration of bridge deck panels still is a problem which has not been satisfactorily resolved.
Recently, a great deal of research has been conducted in an effort to develop means to protect the flexural reinforcing bar matrix in the top half of the panels from the effects of corrosion. The effectiveness of these efforts has been reported in National Cooperative Highway Research Program Report #297(NCHRP 297), Evaluation of Bridge Deck Protective Strategies, September, 1987.
In other known prior art, Mingolla U.S. Pat. No. 4,271,555 and Barnoff U.S. Pat. No. 4,604,841 are both examples of bridge deck panel structures which attempt to overcome certain problems of construction. However, while there are certain novel features to these particular deck panel constructions, both of them use conventional flexural reinforcing steel bar materials near both the upper as well as the lower surface of the deck panel structure.
Other patents which have recently been awarded for bridge deck protection systems, include Jacobs U.S. Pat. No. 4,151,025; U.S. Pat. No. 4,708,888; and Marzocchi, U.S. Pat. No. 4,319,854. They teach, respectively, a membrane barrier system, an electro-chemical xe2x80x9ccathodic protectionxe2x80x9d system, and a combination membrane and electro-chemical system.
Through various research efforts, it has been found that transverse cracking generally occurs at the top surface of the panel substantially directly over the layer of transverse flexural reinforcing re-bars which are in the top half of a bridge deck panel. Such cracks are a significant factor in the deterioration of bridge deck panels, since, as already noted, they allow salts, other corrosive elements, and water to reach the flexural reinforcing bars which are in the top half of the panel and cause them to corrode, thereby accelerating deterioration of the panel. Surprisingly, these cracks form at about right angles to the direction that they would be expected to form if they were due to the stresses caused by the predicted bending moments to which the panel is subjected. However, it is now noted that the observed crack patterns are consistent with tensile stresses due to concrete shrinkage and the effects of temperature changes. This indicates that the control of the formation of transverse cracks directly over the top transverse reinforcing bars due to concrete shrinkage and temperature changes at the surface of bridge deck panels is of paramount importance in avoiding deck panel deterioration. However, effective means for its avoidance are not known to have been previously proposed.
It is well known that the use of either fibers or fabric serves to effectively control upper surface cracking due to volume changes from temperature and shrinkage in structural plain concrete. Such reinforcement materials can be used, in at least the concrete which forms the uppermost portion of a bridge deck panel, to control surface cracking caused by temperature shrinkage changes. It does not require careful control of the concrete mix, nor careful placement of the concrete in order to be successful. Romauldi U.S. Pat. No. 3,429,094 and Kobayashi U.S. Pat. No. 4,565,840 teach the use of fiber reinforcement materials for crack control in concrete. The use of various fiber materials for the reinforcement of concrete is discussed in the Manual of Concrete Practice, ACI. The use of fiber reinforcement materials to restrain cracking due to changes from temperature shrinkage has now become more common than the well established practice of using steel welded wire fabric reinforcement materials for such purposes, see Romauldi U.S. Pat. No. 3,429,094. Fiber or welded wire fabric reinforcing for the purposes of temperature and shrinkage crack control is not used in a sufficient quantity to increase the flexural strength of the concrete, and does not bring it to a level which is defined as xe2x80x9creinforced concretexe2x80x9d. Shrinkage and temperature crack control reinforcing means such as fibers and welded wire fabric thus used, should not to be confused with, nor considered to be xe2x80x9cflexural reinforcing materialxe2x80x9d or xe2x80x9cflexurally reinforcedxe2x80x9d.
Givens U.S. Pat. No. 3,808,085 describes a reinforced concrete structural member for use as in bridge decking which employs fibers as the upper flexural stress reinforcing means, while retaining conventional steel bar flexural reinforcing means for the lower stress reinforcing. In order to provide the upper flexural stress reinforcing means thought to be necessary, Givens improved upon the art made known by Romauldi, cited above, by utilizing more closely spaced short steel wire fibers in the concrete matrix. Although Givens does improve upon the crack resistance of the upper concrete surface, in order to achieve the presumed to be required flexural strength, this is disadvantageous because a greater volume of expensive steel wire fibers are needed to replace the less expensive steel bar reinforcing utilized in conventional art. A further disadvantage of Givens is that a greater volume of corrodible wire fiber material is thereby placed in the upper portion of the slab where they are readily subject to corrosion. Another disadvantage of Givens is that the concrete with a high volume of wire fiber becomes substantially more difficult to mix and place properly.
Givens does not recognize that stress reversal over the interior girders does not occur in accordance with the heretofore known state of the art. Nor does Givens recognize that the primary cracking problem in the upper surface of bridge decks is associated with temperature and shrinkage cracking and corrosion of the upper flexural reinforcing bars. Thus, Givens claims a reinforced concrete structure with both upper and lower stress reinforcing means, wherein the upper stress reinforcing means are wire fibers. Givens specifically discloses an improvement in which fibrous concrete having the same strength as a conventional structure using steel bar reinforcing means is provided. The present invention, as described below, differs from Givens in that the adverse effects of panel deterioration are avoided by using, in some embodiments, only sufficient fiber reinforcing means to adequately control temperature and shrinkage crack formation utilizing specially formulated plain concrete in the upper portion of the panel.
Structural plain concrete differs from reinforced concrete in that plain concrete is assumed to carry all the flexural tensile bending stress with no stress carried by reinforcing materials that may be present. In accordance with the definition for reinforced concrete in xe2x80x9cBuilding Code Requirements for Reinforced Concrete (ACI 318-89) and Commentaryxe2x80x9d, the concrete is assumed to carry no tensile stress, all tensile stress being carried by the reinforcing bars, so that the flexural load bearing capacity is not considered to be diminished after cracking.
Also noted as of interest are Graham U.S. Pat. Nos. 865,490 and 983,274; Henderson U.S. Pat. No. 1,891,763; Rubenstein U.S. Pat. No. 2,850,890; Naaman U.S. Pat. No. 3,852,930; Schupack U.S. Pat. No. 4,159,361; and Matsumoto U.S. Pat. No. 4,379,870; as well as U.K. Patent 578,036; Japanese Patent 2,141,206; and German Patent 3,342,626. Of these, Graham U.S. Pat. Nos. 865,490 and 983,274 disclose a reinforced concrete slab which is designed and intended for placement on the ground. These references includes reinforcing rods in the bottom half, with the latter of these references including the addition of what appears to be a high volume of short wire sections in the upper portion of the concrete to increase the strength of the slab. Because of the size and volume of the wire sections they are added by placing them on top of the concrete and allowing them to settle into the concrete rather than being mixed with the concrete. Graham neither teaches nor suggests a load bearing panel intended to be placed on two or more spaced apart supports, and in the more than eighty years since its filing, its application to load bearing panel construction technology is not known to have occurred.
Schupack U.S. Pat. No. 4,159,361 discloses cold formable, reinforced panel structures which include shrinkage and thermal reinforcement fibers. Schupack neither teaches nor suggests a load bearing panel which is intended to be placed on two or more spaced apart supports, nor a panel which includes flexural reinforcing material, and its application to load bearing panel construction technology is neither taught nor suggested. Matsumoto U.S. Pat. No. 4,379,870 discloses a specific form of synthetic resin reinforcement material which has utility in concrete structures, but it neither teaches nor suggests a load bearing panel which is intended to be placed on two or more spaced apart supports, nor a panel which includes flexural reinforcing material, and its application to load bearing panel construction technology is neither taught or suggested.
It is important to here note that xe2x80x9creinforcement materialxe2x80x9d as used throughout this application is different from xe2x80x9cflexural reinforcing material,xe2x80x9d such as traditional steel re-bars.
Accordingly, it is a principal object of the present invention to provide a load bearing concrete panel which is significantly less expensive then existing panels due to the removal of materials which are now used in state-of-the-art load bearing concrete panels without loss of the utility of such panels, and, in fact, with improved durability of the resulting panels.
A further object of the present invention is to provide a method of making load bearing concrete panels which requires less steps and which is significantly less expensive than existing panels due to the elimination of steps which are now used in the state-of-the-art process for producing load bearing concrete panels without loss of the utility of such panels, and, in fact, with improved durability of the resulting panels.
Yet another object of the present invention is to provide a concrete bridge deck panel structure which has sufficient flexural reinforcement to provide the appropriate amount of flexural strength, while also being designed to eliminate or at least significantly impede both the amount and the speed of surface deterioration of the deck panel
Still yet another object of the present invention is to provide a concrete bridge deck panel structure in which structural flexural reinforcing material, such as steel reinforcing bars, are not required in the top half of the panel near the top surface of the panel between the exterior girders.
Another object of the present invention is to provide a concrete bridge deck panel structure in which very little structural flexural reinforcing material composed of steel need be epoxy coated or connected to a sacrificial anode in order to prevent corrosion of such flexural reinforcing material.
It is yet another object of the present invention to provide a concrete bridge deck panel structure in which chlorides from thawing salts and other corrosive materials do not corrode re-bars in the upper half of the concrete panel with the avoidance of a source of significant cracking and deterioration of the top surface of the bridge deck panel.
It is yet another object of the present invention to provide a concrete bridge deck panel structure in which the source of significant cracking and deterioration of the top surface of the bridge deck panel, and consequent less of structural integrity of such a panel, due to corrosion of the steel reinforcing bars in the upper half of the concrete panel from chlorides from thawing salts and other corrosive materials is substantially avoided.
Yet a further object of the present invention is to provide a concrete bridge deck panel structure in which increased concrete volume shrinkage due to the use of richer concrete mixes is avoided.
Still yet another object of the present invention is to provide a crack and corrosion resistant concrete bridge deck panel without reducing the ability of the bridge deck panel to resist moment stresses imposed thereon by traffic loads.
Another object of the present invention is to provide a bridge deck panel which resists cracking at the upper surface of the panel due to concrete volume shrinkage and temperature changes.
A further object of the present invention is to provide a load bearing concrete panel structure having improved structural properties which prevent or reduce deterioration of the top surface of the panel caused by corrosion of flexural reinforcing materials.
It is a further object of the present invention to provide a load bearing concrete panel structure having improved structural properties which eliminate the cracking or deterioration of the top surface of the panel caused by corrosion stress from transverse flexural reinforcing materials.
Still yet another object of the present invention to provide a concrete bridge deck panel structure having improved structural properties which prevent or reduce deterioration of the top surface of the panel due to temperature and shrinkage volume changes at the top surface.
Another object of the present invention is to provide a concrete panel for use in new bridge construction as well as a process for producing such concrete panels and also for use in rehabilitating existing panel structures, which panel design reduces the corrosion characteristics of the top half and top surface of the panel.
Yet another object of the present invention is to provide a concrete panel design for use in new bridge construction and in rehabilitating existing bridge panel structures, which panel design inhibits deterioration of the top surface of the panel due to temperature and shrinkage volume changes at the top surface.
The invention being taught is a load bearing concrete panel structure which uses structural plain concrete for at least the upper portion of the panel, which concrete has, in preferred embodiments, been specially formulated and installed in a manner to resist temperature change and concrete shrinkage cracking at the upper surface, and which relies on flexural reinforcing materials, such as standard flexural reinforcing bars, being confined to the lower half of the panel to carry superimposed loads.
As discussed in detail above, substantially all known efforts previous hereto to reduce the problem of the corrosion of flexural reinforcing materials have been defensive in nature. That is they have either sought to isolate top flexural reinforcing material from corrosive compositions, for example by the provision of a greater amount of concrete top cover or a water proof membrane on the concrete above the top flexural reinforcing re-bars, or by epoxy coating the re-bars, or they have used electro-chemical methods, such as cathodic protection. However, these solutions do not deal with or solve what is now recognized by the present invention to be a two-fold problem with existing bridge deck panel designs. It is now recognized that problems of panel deterioration and top surface cracking are caused by the flexural reinforcing materials, such as corrodible re-bars, which are located within the top half of the concrete panel, and especially such flexural reinforcing materials which are near the top surface of the panel, and oriented transversely, and which are often coated with epoxy. This is due to the fact that the flexural reinforcing materials which are in the top surface of the panel are subject to corrosion and accelerate degradation of the surface of the panel, and those which are near the top surface of the panel and oriented transversely have now been determined to accelerate the widening and increase the severity of cracks in the top surface due to temperature and concrete shrinkage changes.
Having recognized the above enumerated problems, the present invention, suggests new solutions which are quite different from the defensive solutions utilized in prior and current deck panel designs. It is now postulated that the current practice of placing corrodible flexural reinforcing materials, such as steel re-bars, in the upper half of a concrete bridge deck panel, and especially transversely oriented flexural reinforcing materials which are near the top surface of the panel, is far more detrimental than beneficial to the long term performance and life of the panel. It is therefore concluded that the use of flexural reinforcing materials, and especially of steel reinforcing bars in the top half of a bridge deck panel, as currently practiced, adversely affects the durability of the panel.
Elaborating, this postulate is based on the facts and assumptions that: 1) transversely oriented flexural reinforcing materials, such as reinforcing bars, apparently contribute to increased transverse crack formation due to temperature induced concrete shrinkage at the surface of the panel; 2) when corrodible flexural reinforcing materials in the upper half of a bridge deck panel are exposed to corrosion causing materials and solutions, they corrode and thereby accelerate the deterioration of the surface and the top half of the panel; 3) flexural reinforcing materials, are not required in the top half of a panel for structural strength of the panel; and 4) under standard practices, adequate amounts and distributions of flexural reinforcing materials are present in the bottom half of the panel to provide sufficient flexural strength to the panel.
It has therefore now been discovered, in accordance with the present invention, that the placement of transverse reinforcing bars in the upper portion of bridge deck panels is not required between the exterior supports of concrete panels continuous over two or more supports to provide adequate structural strength to such panels, and that the top layer of longitudinal flexural reinforcing re-bar is not effective in controlling cracking of the upper surface. It has further been discovered, in accordance with the present invention, that the placement of any flexural reinforcing materials in the upper half of bridge deck panels is not required in the region between the exterior support beams to provide adequate structural strength to such panels. It is further postulated that various crack control practices at the upper surface of deck panels should be the governing design criterion for crack control at the top surface of bridge deck panels, and that flexural reinforcing materials should be confined to the lower portion of the bridge deck panel.
Crack control of the upper surface of the deck panels can be further improved using several practices. First, and most preferably, concrete mix compositions can be used which resist surface cracking associated with changes due to temperature and shrinkage design. properties, and such concrete compositions should be the subject of careful placement practice and curing. A second manner of improving crack control at the upper surface of a deck is by the use of fibrous reinforcement materials, preferably in the upper quarter to one-half of the panel. A third manner of improving crack control at the upper surface of a deck is by the use of reinforcement fabric in the uppermost region of the panel in order to resist shrinkage change due to temperature. A small volume of steel welded wire fabric is typically used for this purpose. For best crack control reinforcement, in accordance with the present invention, fiber or fabric reinforcement materials should be placed as close to the upper surface as practicable, preferably no lower than about one-sixth of the total depth of the concrete panel. For bridge deck panels which are 7xc2xd to 9 inches thick, this is typically less than 1xc2xd inches from the surface.
Since it has been determined by the present invention that bridge structures, as they are presently being designed, are in fact being over-designed by the inclusion of excess flexural reinforcing material in the upper portion of the panel; and since it has been further determined that top flexural reinforcing material placement, in accordance with current practice, adversely affects corrosion resistance and crack formation; it has therefore now been discovered that the flexural reinforcing material in the top half of existing bridge deck panel structures can be entirely removed without reducing the strength of the panels below what is sufficient to meet the demands which they must be designed to meet. It has been determined that with flexural reinforcing material in only the lower half of a bridge deck panel, more than sufficient flexural strength for moment bending""stresses of the panel will be provided. It will be readily appreciated that the removal of the two top layers of flexural reinforcing material from the panel will result in substantial reductions in production steps and in the cost of materials and the overall cost of construction.
It is therefore now taught that bridge deck panels with a flexural reinforcing material re-bar matrix in only the lower half of the panel, in accordance with the practice of the present invention, and preferably substantially no flexural reinforcement material, in the upper half of the bridge deck panel have substantially improved durability. A bridge deck panel with the top portion of the deck panel constructed in accordance with the current teaching does not require an extra thickness of concrete cover, or of the other expensive prior art defensive measures, thus, simultaneously, achieving both great cost savings and improved panel durability.
Therefore, to achieve the foregoing and other objects, and in accordance with the purposes of the present invention, a new and improved concrete panel design for use as a bridge deck panel in a bridge structure, or the like is disclosed. The panel design includes at least one layer of concrete which has flexural reinforcing material disposed only within about the lower half, and preferably in the lower one-third to about one-sixth of the concrete panel. The flexural reinforcing material may be even lower if the applicable codes will allow it. In preferred embodiments, a minimum of reinforcement material, such as fiber or fabric may be disposed in the panel, preferably in about the upper one-third to one-half portion of the concrete layer to provide control of cracking due to temperature change and concrete shrinkage.
In an alternative embodiment, a small amount of widely spaced flexural reinforcing bars, preferably oriented in the longitudinal direction, may be used in the upper half of a panel to reduce surface cracking from temperature change and concrete shrinkage.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description, showing the contemplated novel construction, combination, and elements as herein described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiments of the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art.